1. Statement of the Technical Field
The present invention relates to the field of radomes, and more particularly to low loss broadband radomes.
2. Description Of The Related Art
Radomes are dome-like shells that are substantially transparent to radio frequency radiation. Functionally, radomes can be used to protect enclosed electromagnetic devices, such as antennas, from environmental conditions such as wind, solar loading, ice, and snow. Conventional radome types include sandwich, space frame, solid laminate, and air supported.
Radomes can sometimes form very large domes for protecting large electromagnetic devices. For example, a space frame radome installed at Mt. Hebo in 1966 as part of the ballistic missile early warning system, had a diameter of 140 feet. Manufacturing and transportation considerations necessitate that large radomes be constructed in segments, often referred to as panels. One technique for connecting adjacent panels is to construct a rigid domne-supporting frame and subsequently attach a plurality of panels to the frame. Alternately, interlocking radome panels can form a self-supporting shell with adjacent panels connected through some connective mechanism, such as a flange.
Sandwich radomes are rigid, self-supporting structures constructed of numerous doubly curved panels that, after assembly, form a spherical dome. Panels are partially composed of tight tolerance controlled dielectric materials, such as pre-impregnated fiberglass. When panels contain multiple layers of such dielectric materials, foams can be utilized between the layers to maintain spatial relationships. For example, a CFC-free, closed cell, polyisocyanurate foam is often used between dielectric layers. While the frame formed at panel junctions can provide support between adjacent panels, the panels themselves are the primary support infrastructure for the radome. Notably, the panel segments of a sandwich radome are randomly oriented to significantly reduce the boresight error and sidelobe perturbations that can form at panel junctions.
Another radome type is a space frame radome. A space frame radome is a rigid, self-supporting structure containing a load bearing frame and wall members supported by the frame. The frame is composed of triangular panels assembled into a geodesic dome using quasi-random geometric placement to optimize electromagnetic performance. Notably, the losses caused by a radome frame, often referred to as scatter loss, can be several times greater than a wall insertion loss of the radome, which signifies the loss caused by reflections occurring at the panel/air boundary. The wall of a space frame radome can be a thin electromagnetically transmissive membrane material, such as ESSCOLAM®. Typical materials for forming the frame of a space frame radome can include dielectrics, such as fiberglass, and metals, such as aluminum and steel.
When metals are used in the frame construction, electromagnetic waves striking the frame are always reflected. Dielectrics, such as fiberglass, which reflect partially electromagnetic radiation, can be beneficially utilized in the frame formation. Unfortunately, dielectric frames have numerous drawbacks. Structural characteristics of dielectric materials require a substantially greater cross sectional area than equivalent metallic counterparts in order to provide equivalent mechanical support. Furthermore, the width of conventional dielectric materials affect the wave transmissions as can be shown using transmission line analysis techniques. Accordingly, radio frequency wave perturbations can sometimes be greater when using a dielectric frame than the equivalent perturbations resulting from a metallic frame.
Solid laminate radomes are rigid, self-supporting structures that use doubly curved panels to form a truncated spherical dome. The panels of a solid laminate radome can be constructed from pre-impregnated fiberglass and are generally arranged in a regular or “orange peel” geometry. Solid laminate designs can be cost effective for smaller radomes, but are generally unsuitable for larger ones.
A fourth type of radome, an air supported radome, is not generally utilized outside tactical applications or temporary installations. The air supported radome is an active system consisting of a thin fabric envelope and a system comprising a power supply and blowers. The fabric envelope is formed from a flexible dielectric material. The blowers keep the outer fabric envelope inflated in a balloon-like fashion. Because an air frame radome must be inflated at all times, reliable operation generally depends upon non-interruptible power supplies and redundant blower systems.
Radome induced wave perturbations are a principal consideration in radome construction. An ideal radome is electromagnetically transparent to a large number of radio frequencies, through a wide range of incident angles. However, in practice, conventional radomes are inherently lossy and are narrowbanded. Moreover, loss generally increases with angle of incidence. Radomes are generally designed to have a lower loss at a specific angle of incidence and a larger loss at the remaining angles. Often, the angle at normal incidence is chosen as the angle of lower loss in a radome design.
Traditionally, the RF loss in radomes is minimized by adjusting the phase factor of the radome at a single radio frequency. For instance, the thickness of a dielectric radome having a given permittivity can be a multiple of half a wavelength at a given frequency. When so formed, a very small reflection coefficient will result at that frequency. Unfortunately, such a radome transmits electromagnetic waves with minimal loss only over a narrow frequency band about a center frequency. In order to overcome this limitation, some radomes are made of several layers of dielectric slabs, so that a broader group of frequencies can be transmitted with low loss.
In addition, the walls of conventional radomes are formed from dielectric materials which provide a relative magnetic permeability of nearly one. In fact, conventional teachings suggest that metals, including magnetic metals, within radomes, are to be avoided unless required by overriding structural considerations. The reason why magnetic materials have been avoided in the past, in the fabrication of radomes, is the inherent large value of the magnetic loss tangent. The magnetic loss tangent is the driving material property used in the fabrication of microwave absorbers. However, when the magnetic loss tangent is reduced to acceptably low levels, of the order of 0.1 or lower, the relative permeability can be used to reduce the transmission loss by matching the intrinsic impedances of the mediums involved in the wave transmission and reflection phenomenons.